Storage tube with array on pnpn diodes

ABSTRACT

A storage tube is provided with a storage element comprising a plurality of PNPN bistable photodiodes which share a common terminal P-type zone, the other zones being discrete. A first electrode makes low resistance connection to the common P-type zone and a second electrode makes electrical connection to the discrete terminal N-type zones by way of individual film resistors. The radiation pattern to be stored is made incident on the P-type surface and an electron beam scans the opposite surface. The operating potentials are such that the beam makes impact only on those N-type terminal zones associated with photodiodes triggered to the low resistance state by the incident radiation.

United States Patent Amelio et a1.

[54] STORAGE TUBE WITH ARRAY ON PNPN DIODES [72] Inventors: Gilbert Frank Amelio, Basking Ridge; George Elwood Smith, Murray Hill, both of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorpornted, Murray Hill, Berkeley 1-leights,N.J.

221 Filed: Mareh3, 1970 1211 Appl.No.: 16,197

[56] References Cited UNITED STATES PATENTS 3,355,669 11/1967 Avins ....'317/23s E 3,391,035 7/1968 Mackintosh ..3l7/235 E 3,419,746 12/1968 Crowell 61 al. ....313/65 AB Primary Examiner-Robert Segal Attorney-R. .l. Guenther and Arthur J. Torsiglieri [57] ABSTRACT A storage tube is provided with a storage element comprising a plurality of PNPN bistable photodiodes which share a common terminal P-type zone, the other zones being discrete. A first electrode makes low resistance connection to the common P-type zone and a second electrode makes electrical connection to the discrete terminal N-type zones by way of individual film resistors, The radiation pattern to be stored is made incident on the P-type surface and an electron beam scans the opposite surface. The operating potentials are such that the beam makes impact only on those N-type terminal zones associated with photodiodes triggered to the low resistance state by the incident radiation.

2 Claims, 2 Drawing Figures PAT-ENTEDHETIH m2 3.701.914

GE AMEL/O /NVEN7'0R5 GE SMITH BY ,&

ATTORNEY 1 STORAGE TUBE WITH ARRAY ON PNPN DIODES BACKGROUND OF THE INVENTION This invention relates to an electron beam storage tube capable of storing an electrical representation of an image scene or a signal pattern in a tow-tone scale.

Storage tubes which provide storage of an image scene in a two-tone scale have a variety of applications. Such tubes, for example, are particularly useful in systems for recording and transmitting information representative of black and white characters in a document.

A variety of such tubes have been proposed hitherto but none is without some disadvantages. Advantages particularly desirable include high sensitivity to permit operation with low light levels, ruggedness to permit long life, and a reasonable cost. It is also advantageous for some applications if the stored information is capable of nondestructive readout to permit multiple readings.

SUMMARY OF THE INVENTION To these ends, one feature of the invention is a storage element or target which advantageously is of an elemental semiconductor, such as silicon, which is rugged and whose technology is now well developed. This target is provided with an array, typically a twodimensional matrix, of PNPN diodes of the kind displaying bistable properties. Each diode of the array includes a common first terminal zone, a pair of discrete floating intennediate zones, and a discrete second terminal zone. Each of the discrete terminal zones is separately connected through an individual thin film resistor to a common conductive electrode. The array I can be viewed as made up of a plurality of bistable PNPN photodiodes of the kind described in US. Pat. No. 2,855,524 which issued Oct. 7, l958 to W. Shockley. The image scene is made incident on the surface including the large-area P-type common first terminal zone and an electron beam scans the surface including the array of discrete N-type second terminal zones. I

Each diode serves as a separate bistable cell which assumes one or the other stable state depending on the intensity of the incident radiation penetrating to its floating intermediate zones. The use of the thin film resist'ive network permits isolation without need of an elaborate electrode arrangement. Detection of the state of a particular diode is provided by scanning the surface corresponding to the second terminal zone of the diode under bias conditions that the electron beam makes impact on a diode only if it is in the low resistance state.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawing:

FIG. 1 illustrates the basic elements of a typical storage tube; and

FIG. 2 shows a fragmentary view of a target in accordance with the invention for use in the storage tube of FIG. I.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS With reference now to FIG. 1, the vacuum envelope 11 includes at opposite ends an electron gun l2 and a target 13. In conventional fashion, the electron gun includes an electron-emissive cathode and an electrode system for forming the emitted electrons into a beam and accelerating the beam towards the target. Intermediate the gun and the target a deflection system (not shown) is used to deflect the beam across the front of the target in a desired scanning pattern. Usually the pattern is the same as is used in home television. The image pattern to be stored is made incident on the back of the target. Typically, a lens system is used to focus the image scene on the back of the target. Advantageously, a grid 15 is positioned close to the front of the target and is maintained at a positive voltage with respect to the cathode of the electron gun. This grid serves to insure that the electron beam is incident normal to the target over its whole surface.

The details of a target 13 suitable for use in the invention are illustrated in FIG. 2. A semiconductive wafer, typically of monocrystalline silicon, comprises a common P-type first terminal portion 21 which includes a planar surface portion 21A and a grid-like portion 218 which extends to the opposite surface and serves to define a two-dimensional array of discrete N- type zones 22. Within each zone 22 is nested a P-type zone 23 and within each of the latter is nested an N- type surface zone 24 which serves as the second terminal zone. An insulating layer 25, typically of silicon dioxide covers the front surface of the wafer except for openings provided therein to expose the central portions of surface zones 24. A conductive grid 26 is formed over the silicon oxide layer which surrounds the exposed N-type zones 24. A thin film 27 of a resistive material, such as high resistivity polycrystalline silicon, then is deposited over the surface, effectively connect ing each of the discrete N-type zones to the grid 26 by way of a high resistance provided by the film resistance. A resistance of 10' ohms is typical, although this resistance may be in the range of 10 to l0 ohms.

In operation, the conductive grid 26 is maintained at ground potential, corresponding to the potential of the cathode, and the common P-type terminal zone is maintained at a positive potential by virtue of an electrode28, which typically is a ring around the periphery of the wafer, making a low resistance connection to the zone 21. The output signal is derived as the voltage developed across the load resistor 29 which is connected between electrode 28 and the voltage source 30 used to establish the positive potential on electrode 28. I

The positive voltage applied by source 30 is such that there is developed a reverse bias across each of the P junctions between N-type zones 22 and P-type zones 23 which is less than the breakdown voltage of the junction in the absence of light incident on the corresponding diode.

Looking at a particular cell or diode in the absence of any light falling thereon, its reverse biased junction is a very high resistance, and so very little current flows through such cell. Hence, there is only a negligible drop across its thin film resistor and the associated Ntype zone 24 is essentially at ground or cathode potential. As a consequence, when the scanning beam is aimed at this N-type zone, substantially no electrons will land and no pulse is detected across the load resistor 29.

However, in the manner characteristic of PNPN photodiodes, diodes of the kind described in the Shockley patent, when light of an intensity above a controllable threshold is incident on a diode and sufficient hole-electron pairs are generated in zones 22 and 23 that avalanching will occur, the diode will assume its low resistance state. There will then be significant current flow through it and its associated thin film resistor, and a rise in the potential of the associated N-type zone will follow. Now when the scanning beam is aimed at this diode, electrons will land efficiently and a current pulse will be developed across load resistor 28 while the beam is incident on this diode.

As is further characteristic of such diodes, once in a low resistance state, a diode will remain in that state so long as there is applied a voltage sufficient to maintain a current above a threshold sustained value. The voltage source 30 is adjusted to maintain such a flow through such an element and there consequently will be a steady flow of current through the load corresponding to the sum of the individual currents flowing through all the diodes in a low resistance state. To this end, advantageously the utilization circuitry is designed to respond to changes in the current flow through the load. A blocking capacitor 31 inserted in series with the output terminal serves this purpose.

It is also desirable ordinarily, for keeping the noise 7 content of the output current low, to provide that the diodes not change states during the reading process since a change in state introduces a change in the current flow through the load resistor.

This should not be a real problem since ordinarily the invention will be used to store an image which is not changing significiantly in the course of a scanning cycle. If the image does change, the nature of our tube is such that a diode once irradiated above threshold will continue in a low resistance state until erasure.

Erasure can be readily achieved by temporarily turning off the voltage supplied by source 30, as by an electronic switch periodically shunting such source, thereby reducing the current flow through any diode below the sustaining value. Alternatively, erasure can be achieved by scanning at a beam voltage above the crossover point for secondary emission and at a beam current sufficiently high that the charge stored is removed by way of the secondary electrons.

Moreover, it should be evident that the storage tube described can be used to store a pattern of other forms of suitable radiation, such as X-rays or electrons, which can be made to penetrate to the intermediate zones of the diodes and there generate enough hole-electron pairs to switch the resistance state of the diode.

The conditions for switching PNPN photodiodes are set forth in the Shockley patent and are well known so these do not warrant detailed discussion here.

There are a variety of design considerations. First it is important that the characteristics of each of the various diodes be such that each can operate as a bistable PNPN photodiode. in particular the ionizing radiation involved should penetrate adequately the common P- type terminal zone and reach the intermediate zones for creating hole-electron pairs therein in sufficient numbers to permit switching of the resistive state of the diodes. This is facilitated by making the common P- type terminal zone thin. Actually, it may be usually advantageous to eliminate portion 21A entirely leaving only the honeycomb portion 213 to serve as the common terminal zone. The intermediate zones should also be thin compared to the diffusion length of minority carriers therein for reliable switching. it is also important that the scanning electron beam penetrate to the N-type terminal zone when the particular diode is in a I low resistance state and so there should be avoided covering the surface of each such N-type terminal zone with too thick a resistive film. A thickness of about 1,000 Angstroms is suitable. A variety of materials including germanium and gallium arsenide should also be useful. The size and spacing of the discrete N-type terminal zones is chosen to be consistent with the resolution and quality desired. For many applications it will be unnecessary to have a high degree of resolution so that a coarse spacing is feasible. In some instances, it may be desirable to utilize a beam diameter small compared to the size of the N-type islands and to provide a feedback for improved registration. in other instances, it may be advantageous to utilize a beam diameter large relative to the islands size and get an averaging effect.

A variety of ways can be used for the fabrication of the target. Typically these involve ion implantation, diffusion, oxide masking, epitaxial growth, and photolithographic techniques now well known for the fabrication of integrated circuits.

What is claimed is:

l. A storage tube of the kind comprising an electron beam source and a storage element in target relationship Characterized in that the storage element comprises T a semiconductive monocrystalline silicon wafer having a pair of opposed major surfaces and comprising a P-type terminal zone extending between the opposed major surfaces, a plurality of N-type intermediate zones disposed within the P -type terminal zone, a like plurality of P-type intermediate zones, each P-type intermediate zone nested within a different one of the N-type intermediate zones, and a like plurality of N-type terminal zones lying on one of the major surfaces, each terminal N-type zone nested within a different one of the P- type intermediate zones whereby there is formed a two-dimensional array of bistable PNPN diodes sharing a common P-type terminal zone and having discrete remaining zones, the surface including the discrete N-type terminal zones being oriented to be scanned by the electron beam and the opposed surface being oriented to be irradiated with a pattern to be stored,

a first electrode making low resistance electrical connection to the common P-type terminal zone;

an insulating layer of silicon dioxide over the surface including the N-type terminal zones and, second electrode comprising a conductive grid overlying said insulating layer and making electrical connection to each of the N-type terminal 2. A storage tube in accordance with claim 1 in which the film is polycrystalline silicon. 

1. A storage tube of the kind comprising an electron beam source and a storage element in target relationship Characterized in that the storage element comprises a semiconductive monocrystalline silicon wafer having a pair of opposed major surfaces and comprising a P-type terminal zone extending between the opposed major surfaces, a plurality of Ntype intermediate zones disposed within the P-type terminal zone, a like plurality of P-type intermediate zones, each Ptype intermediate zone nested within a different one of the Ntype intermediate zones, and a like plurality of N-type terminal zones lying on one of the major surfaces, each terminal N-type zone nested within a different one of the Ptype intermediate zones whereby there is formed a twodimensional array of bistable PNPN diodes sharing a common Ptype terminal zone and having discrete remaining zones, the surface including the discrete N-type terminal zones being oriented to be scanned by the electron beam and the opposed surface being oriented to be irradiated with a pattern to be stored, a first electrode making low resistance electrical connection to the common P-type terminal zone; an insulating layer of silicon dioxide over the surface including the N-type terminal zones and, a second electrode comprising a conductive grid overlying said insulating layer and making electrical connection to each of the N-type terminal zones by way of a high resistance silicon film on the insulating layer extending between the conductive grid and the discrete N-type terminal zones and inserting a resistance of between 105 and 109 ohms between the conductive grid and the terminal zones.
 2. A storage tube in accordance with claim 1 in which the film is polycrystalline silicon. 